Many women are unfortunate enough to require long term heparin prophylaxis in pregnancy. Standard heparin doesn't cross the placenta or pose a threat to the fetus. Long term use of heparin frequently has adverse effects for the mother such as bleeding, osteoporosis & thrombocytopenia. A new low molecular weight heparin (LMWH) has been shown to be effective in preventing deep venous thrombosis and appears safe in pregnancy as it doesn't cross the placenta. The study will elucidate whether there are adverse effects from long term LMWH use in pregnancy.